Electrical energy conversion between DC and AC is the fundamental process involved in energy generation (photovoltaic inverters), energy distribution (High Voltage DC power links) and producing derivatives in other forms of energy such as mechanical (powering electrical motors), heating (induction ovens etc.), communications (radio transmitters) and special purposes like generating the high power high frequency pulses for nuclear magnetic resonance (NMR) tools for oil and gas well logging. Each field of application has specific requirements depending upon the operational frequency, output power and voltage levels which defy use of proper semiconductor switches—thyristors, GTO, MOSFET or IGBT type for maximum conversion efficiency affecting both total system losses and particularly power switches losses resulting in components temperature rise and hence decreased reliability.
In most cases output AC voltage has to be maintained stable within the narrow margin while DC input voltage may change in a wide range depending on the original energy source; this statement is fully applicable to the renewable energy conversion. In other applications such as radio transmitters and NMR transmitters the output AC or RF voltage is modulated with the amplitude/RF envelope forming signal with zero minimum amplitude. In both cases the definition of AC assumes the output to be the sinusoidal voltage delivered to the load with low level of unwanted higher harmonics.
High power DC to AC converters may include multiple modules (multimodule converters—MMC) with means to combine their outputs thus distributing maximum input and output voltages, dissipated power and optimizing output signal waveform (spectrum control). The industrial frequency (50 Hz/60 Hz) converters and electrical motor drives may use switches operating at higher frequencies (carrier frequencies up to few kHz) and form the output signal using single or multilevel pulse width modulation (PWM). If the carrier frequency is significantly higher than the converter output frequency (for example 60 Hz) the byproduct of the process of modulation carrier (for example 2 kHz) high frequency harmonics may be easily filtered out with the output series filter with low losses for the fundamental frequency 50 Hz/60 Hz.
Another approach in DC to AC conversion is to produce sine voltage using power switches operating with the fundamental frequency and to use a phase shift modulation (PSM) to form a “ladder voltage”. Both methods may fully control shape and amplitude of the output voltage. But while the first method is not applicable to the high frequency operation due to the switching losses which are proportional to the switching frequency (carrier frequency) the second method involves uneven conduction time resulting in uneven power dissipation of individual modules. Additional disadvantage of the traditional “ladder” method is changing of the output voltage spectrum in the process of amplitude regulation and need of the complicated control circuitry computing proper timing for each module output pulse in real time based on input DC power bus voltage.
For high frequency applications the amplitude control/modulation method known as Chireix outphasing method is based on combining two sine voltages (two original vectors) with the stable amplitude and forming a sine voltage (combined vector) with the amplitude depending on the relative phase shift between them. To produce a sine voltage at the output two sine voltages (original vectors) are required. Chireix method cannot be used without the modifications in conjunction with switch mode modules and resonant loads due to presence of higher harmonics resulting in extremely high output currents charging and discharging the capacitance of the resonant load.
Proposed by P. Wilkinson Dent method of exciting the resonant loads includes generating two sequences of the rectangular pulses and combining them with the proper relative phase shift (outphasing method of vector combining) exciting the resonant load. A necessary attribute for this combining is a coupling line or filter connected between the resonant load and each of two sequence sources. This filter, preferably tuned to the fundamental frequency, is transparent to the fundamental frequency but limits the higher harmonics. Vector outphasing method provides an efficient way to generate high frequency sine signal with fully regulated amplitude. On the other side adding the output filter creates another problem related to the “saddle” frequency response of the combination of series and parallel resonant tanks forming a band pass filter. The transferring function in close vicinity of the fundamental frequency has to be flat enough to prevent spectrum of the modulation signal from distortion but at the same time higher harmonics of the rectangular pulses at the input of the filter may be amplified by side saddle horns and increase unwanted current of the converter output.
Method described in US 2013/0176140 A1 propose a mitigation of third harmonic problem. This method and topology simply remove third harmonic from each of two inverter outputs before combining. Third harmonic is removed from the spectrum of the output voltage using two additional modules with the output signals shifted 60° (π/3) in reference to the original modules. Other higher harmonics still presenting in the output voltage spectrum may still produce excessive heating, reducing total DC to AC conversion efficiency and decreasing expected lifetime of the power components. According to Arrhenius equation life expectancy is decreased twice for every additional 10 deg C. of the temperature rise. This factor is extremely important for the equipment operating in the harsh high temperature environment such as NMR tools used for oil and gas industry well logging. Any improvements in removing or keeping low unwanted harmonics in the converter output voltage have significant positive impact on the converter/transmitter reliability and operational and maintenance cost.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure, but on the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed with the given embodiments by the scope of the appended claims.